There are many reasons for the torque fluctuation of brushless DC motors, which can be divided into the following aspects: torque fluctuation caused by the electromagnetic torque generation principle, torque fluctuation caused by current commutation, and torque fluctuation caused by the cogging effect. . In addition, there are armature reactions and torque fluctuations caused by motor process defects. The breakdown is as follows:

1. Non-ideal back electromotive force waveform causes principle electromagnetic torque fluctuations

From the working principle, sine wave drive is a high-performance control method. The current is continuous. The three-phase sine wave AC current and the three-phase sine wave counter electromotive force in the three-phase winding work together to produce a smooth and stable electromagnetic torque. In theory, uniform output torque can be obtained regardless of the rotation angle, and a well-designed system can achieve low ripple torque of less than 3%. The square wave drive stator magnetic field is a discontinuous, step-by-step rotation. From the principle of electromagnetic torque generation, the torque fluctuation of the brushless DC motor is much larger than that of the sine wave drive. Although in the ideal case where the back electromotive force is a trapezoidal wave, the flat-top width is 120° electrical angle, and the stator current is a square wave, the electromagnetic torque generated will be a constant value without considering the commutation process, and theoretically there will be no torque fluctuation. However, in actual motors, due to design and manufacturing reasons, it is difficult to achieve a back electromotive force with a trapezoidal waveform with a flat top width of 120° electrical angle; in fact, the back electromotive force waveform of most brushless DC motors cannot be a trapezoidal waveform. , and is closer to a sine wave; in this way, the current waveform will inevitably deviate from the square wave, and these non-ideal conditions will lead to principled fluctuations in its electromagnetic torque.

2. Torque fluctuations caused by commutation

When the brushless DC motor is working, the stator windings commutate in a certain sequence. Even under the ideal situation where the back electromotive force is a trapezoidal wave with a flat top width of 120° electrical angle and the stator current is a square wave, due to the inductance of the phase winding, there is a commutation period between each two states. There is a transition process when the current switches from one phase to another. The lag of the current change causes obvious fluctuations in the electromagnetic torque generated during commutation, which is called commutation torque fluctuation.

3. Torque fluctuations caused by cogging effect

When the stator core of the brushless DC motor has cogging, the air gap is uneven due to the presence of stator cogging, so the air gap magnetic permeability is not constant. When the rotor is at different angles, the air gap magnetic field will change, producing cogging torque. Cogging torque is related to the rotor position, thus causing torque ripples. Cogging torque is an inherent characteristic of hydromagnetic motors. When the motor is running at low speed and light load, cogging torque will cause obvious speed fluctuations, and produce vibration and noise. Therefore, how to weaken the cogging torque is one of the more important goals in the design of permanent magnet motors.

The causes of cogging torque are different from the two aforementioned causes of torque fluctuations. The two causes of torque fluctuations mentioned above are both caused by the interaction between the stator current and the rotor magnetic field, while the cogging torque is generated by the interaction between the stator core and the rotor magnetic field. A better way to eliminate cogging is to use a slotless motor structure. Regardless of the form of the armature winding of a slotless motor, its thickness is always part of the actual air gap. Therefore, the actual equivalent air gap of a slotless motor is much larger than that of a slotted motor, and the required excitation magnetomotive force It is also much larger, which limited the capacity and development of slotless motors in the early days. In recent years, with the rapid development of magnetic materials, especially the application of high magnetic energy accumulation rare earth permanent magnet materials such as neodymium iron boron, conditions have been created for the practical use of slotless motors. The slotless structure, because it also has a large air gap, can not only completely eliminate the torque fluctuations caused by the cogging effect, but also greatly weaken the torque fluctuations caused by armature reaction and mechanical eccentricity.

4. Torque fluctuations caused by armature reaction

The influence of the armature magnetomotive force on the main magnetic field of the air gap permanent magnet is called the armature reaction. The armature reaction of brushless DC motor is relatively complex. The armature reaction magnetomotive force will distort the air gap main magnetic field waveform. The magnetic flux density of the air gap main magnetic field is no longer the square wave at no load, and the back electromotive force will also be distorted, causing torque fluctuations. Most modern brushless DC motors use high-performance rare earth permanent magnet materials. If the tile-shaped surface mount type is used, the armature reaction will have a weak impact on the main magnetic field of the air gap. This is because the armature reaction magnetic circuit has to pass through the air gap and the permanent magnet. The magnetic permeability of the permanent magnet material is very close to the magnetic permeability of the air. This makes the armature reaction magnetic circuit have a very large magnetic resistance. The magnetic flux of the armature reaction is very small, and its influence on the main magnetic field of the air gap is negligible. However, for the built-in rotor structure, the influence of armature reaction cannot be ignored.

5. Torque fluctuations caused by motor machining defects and material inconsistencies

Machining defects and material inconsistencies are also one of the important causes of torque fluctuations in brushless DC motors. For example, the dimensional and shape deviations generated during motor machining and assembly, the uneven distribution of slots in the stator punching, the eccentricity of the inner and outer circles of the stator, the unilateral magnetic pull caused by the coaxiality deviation of the stator and rotor; friction in the bearing system Uneven torque; torque fluctuations caused by inaccurate positioning of the rotor position sensor; torque fluctuations caused by asymmetric parameters of each phase winding and differences in performance parameters of electronic components; materials of each part in the magnetic circuit, especially each magnetic pole, are permanently Torque fluctuations caused by inconsistent magnet performance, etc. Therefore, improving the level of processing and manufacturing is also an important measure to reduce torque ripple.

Research shows that when the motor is under no load or light load, the main torque fluctuation is caused by cogging torque, while under load, the main torque fluctuation is caused by electromagnetic principle torque fluctuation and commutation. Therefore, it is of great significance to analyze the causes of torque fluctuations and study methods to reduce or suppress torque fluctuations.